update NAS-Identifier in Access-Request before proxying to other radius server

[C.F. Yeung]

I want to update NAS-Identifier in Access-Request before proxying to other
radius server. Tried adding the following lines in default but in vain. So,
where should I put the following unlang?

        update request {
                NAS-Identifier = "new-nas-identifier"
        }

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